Electro-acoustic transducer

ABSTRACT

This invention relates to electro-acoustic transducers such as is generally indicated at 10 and which comprise a body 11 defining a chamber 12, a plate 13 bearing a micro-phone or speaker 14 to which is attached a stalk 15, a diaphragm 16, connectable to the stalk 15, which extends substantially across the chamber 12, an end piece 17 and an end cap 18. The end piece 17 has a kidney-shaped recess 21 which provides the main communication path between subsidiary inlets/outlets 19 and a main inlet/outlet 20. When the channel 22 is closed off by the end cap 18 it is effectively in the form of a tube and constitutes a Thuras tube. The length of this path effectively creates a long discharge path between the wearers&#39; ear and any grounded metal part within the transducer and hence protects against electric shock.

TECHNICAL FIELD

This invention relates to electro-acoustic transducers and particularly,but not exclusively, to such transducers in the form of ear pieces fortelephone headsets and the like.

BACKGROUND PRIOR ART

Telephone operators often build up very substantial static voltages ontheir bodies, particularly if they are imprudent enough to wear man-madefibers. If they then plug their headset into the telephone exchangeapparatus, it is not uncommon for a spark to jump from their ear throughthe acoustic inlets in the ear piece to a grounded metal portion of thetransducer. As these sparks can be derived from several thousands ofvolts, the sensation is at best uncomfortable. This problem particularlyarises in small devices where distances between the acoustic inlets andgrounded portions of transducers are normally extremely small.Additionally, it has been found difficult to produce economically andcompactly a suitable method of boosting the bass response of such earpieces.

The present invention is provided to solve these and other problems.

SUMMARY OF THE INVENTION

From one aspect the invention consists in an electro-acoustic transducerincluding a body defining a chamber, a diaphragm extending across atleast part of the chamber to define front and rear portions, a mainacoustic inlet/outlet connected to the front portion and a Thuras tubeconnecting the rear portion and the main inlet/outlet, wherein at leasta portion of the Thuras tube is defined by an end piece forming a wallof the chamber.

In a preferred embodiment, the end piece further defines the mainacoustic inlet/outlet, and the Thuras tube may be formed as a channel inthat end piece. Particularly conveniently the transducer may furtherinclude a cap overlying the end piece with subsidiary inlet/outletsformed therein and offset from the main inlet/outlet. In this case theend cap may close off the channel to complete the tube.

The end cap and end piece may together define an extended air pathbetween the subsidiary inlet/outlets and a grounded metal part withinthe chamber. Current designs simply have these inlets and outletspassing straight through the cap into the chamber, although sometimesthere are guards to prevent mechanical damage due to penetration throughthese holes.

From another aspect there is provided an electro-acoustic transducerhaving a chamber including a grounded metal part, a non-electricallyconducting cover defining a number of subsidiary acoustic inlet/outletsand a main acoustic inlet/outlet for the chamber wherein the coverdefines an extended air path between the subsidiary outlets and agrounded metal part within.

As has been mentioned above, this extended path reduces the risk ofelectrical discharge occurring along the path typically for potentialdifferences of up to 15,000 volts D.C.

Preferably the chamber is generally cylindrical, the main inlet/outletis at or adjacent the axis of the chamber, the subsidiary inlets/outletsare radially offset therefrom and the main inlet/outlet is defined inthe wall extending across the chamber.

Particularly conveniently the subsidiary inlets/outlets are formed in anend cap which overlies the wall and a part of the air path is definedbetween the air cap and the wall.

Although the invention has been defined, it is to be understood that theinvention includes any inventive combination of the features set outabove or in the following description.

The invention may be performed in various ways and a specific embodimentwill now be described, by way of example, with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an exploded view of an electro-acoustic transducer;

FIG. 2 is a more detailed view from above of an end piece of thetransducer, and

FIG. 3 is a diagrammatic cross-sectional view of the upper part of thetransducer demonstrating a discharge path.

DETAILED DESCRIPTION

While this invention is susceptible of embodiments in many differentforms, there is shown in the drawings and will herein be described indetail, a preferred embodiment of the invention with the understandingthat the present disclosure is to be considered as an exemplification ofthe principles of the invention and is not intended to limit the broadaspects of the invention to the embodiment illustrated.

An electro-acoustic transducer is generally indicated at 10 andcomprises a body 11, defining a chamber 12, a plate 13 bearing amicrophone or speaker 14 to which is attached a stalk 15, a diaphragm16, connectable to the stalk 15, which extends substantially across thechamber 12, an end piece 17 and an end cap 18.

The end cap 18 defines a number of spaced subsidiary acousticinlets/outlets 19, through which sound may pass either to, or from, amain acoustic inlet/outlet 20 defined in the end cap 18. Thisinlet/outlet 20 communicates directly with the diaphragm 16 and hencevia the stalk 15, with the microphone 14.

It will be noted that the end piece 17 has a kidney-shaped recess 21,which provides the main communication path between the subsidiaryinlets/outlets and the main inlet/outlet 20. A channel 22 projects intothis recess 21 to extend from the main inlet/outlet 20 to a verticalshaft 23 which extends through the thickness of the end cap 18 tocommunicate with aligned notches 24,25 in the diaphragm support 26 andthe plate 13.

It will thus be seen that the main inlet/outlet 20 communicates bothwith a front portion 27 of the chamber 12 contained within the diaphragm16 and a rear portion 28 defined between the base 29 of the body 11 andthe plate 13. As the end cap 18 is shaped so that it closes off theupper mouth of the channel 22, the channel 22 is effectively in the formof a tube and constitutes a Thuras tube providing bass resonance orreinforcement from the rear portion 28 of the chamber 12.

This construction is particularly compact, economic to make and easy toassemble.

As has been mentioned above, many traditional ear pieces for headsets,and indeed other transducers, have the sound inlets passing simplythrough the end cap 18, and the end piece 17 does not exist. This meansthat there is a very short discharge path between a wearer's ear and anygrounded metal part within the transducer, such as the diaphragm support26.

Referring to FIG. 3, it will be seen that the interposing of the endpiece 17 and the provision of a central main acoustic inlet/outlet, hasthe effect of making a considerably extended air path between the ear(here represented as an electrode 30) and the metal part 31. The resultis that very high voltages, say typically 14,000 or 15,000 volts D.C.,have to be built up before a spark will jump along a path of thislength. Clearly, in order for this aspect of the invention to operate,the end piece 17 and end cap 18 must be electrically insulating.

It will be appreciated that the provision of both of the extended airpath and the Thuras tube in the end piece/end cap combination isparticularly effective in producing a much improved transducer at lowadditional cost.

Although these advantages have been particularly demonstrated inconnection with ear pieces, they are equally applicable to anytransducer use in which large D.C. voltages are likely to occur adjacentthe sound inlet/outlets.

It will be understood that the invention may be embodied in otherspecific forms without departing from the spirit or centralcharacteristics thereof. The present examples and embodiments,therefore, are to be considered in all respects as illustrative and notrestrictive, and the invention is not to be limited to the details givenherein.

I claim:
 1. An electro-acoustic transducer including a body defining achamber, a diaphragm extending across at least part of the chamber todefine a front chamber portion and a rear chamber portion, a groundedmetal part within the rear chamber, an end piece extending across thechamber and forming a wall of the front chamber portion, the end piecehaving a main acoustic inlet/outlet and a Thuras tube connecting therear portion and the main inlet/outlet, wherein at least a portion ofthe Thuras tube is defined by a channel formed in the end piece formingthe wall of the chamber; and an end cap overlying the end piece withsubsidiary inlets/outlets formed therein offset from the maininlet/outlet, said end cap closing off the channel to form said portionof the Thuras tube, said channel defining an extended air path betweenthe subsidiary inlets/outlets and the grounded metal part.
 2. Thetransducer of claim 1 wherein the chamber is generally cylindrical.